Spatial patterns of HA reduction in three Has mutant mouse models. Tissue staining of HA with biotinylated hyaluronan-binding protein (Laurent et al., 1991) was performed in the forebrain sections of WT (A, E), Has3^−/− (B, F), Has2^CKO (C, G), and Has1^−/− (D, H) mice. Note that in WT mice, HA is strongly expressed in the hippocampus (A). In Has3^−/− mice, hippocampal HA staining is markedly reduced (B). On the other hand, HA staining in the hippocampus is largely preserved in Has2^CKO mice (C). Instead, reduction in HA is prominent in the superficial layers of the cortex and in fiber tracts in Has2^CKO mice (e.g., the corpus callosum and the cingulum bundle; C, asterisk). The HA staining pattern in Has1^−/− mice is essentially indistinguishable from that of WT mice (D). Scale bar, 200 μm. E–H, High-magnification images of the CA1 region. Note that not only is the level of HA staining greatly reduced in Has3^−/− mice, but intercellular HA staining between neurons in the stratum pyramidale is essentially lost in Has3^−/− mice (F). Scale bar, 30 μm.